In this case, it may be trying to raise $ for a cause. I suspect that the con may be getting a cut as well (heck, it may be part of his TNG contract that they get a portion of such things ... esp. if he is in a Star Fleet uniform).

Also, the Con may not be paying an honorarium and this is how he recoups some cost.

Now charging $5 for an autographed picture I can sort of understand because it costs some $ to have the pictures printed (and shipped) and 5s are easier to deal with than the mix of dollars and cents it actually costs.